A micro mirror array based SLM integrates dynamic micromechanical systems with semiconductor circuitry. A reflective SLM can include an array of mirror cells comprised of a mirror, hinge, support structure, and underlying CMOS circuitry. One mechanism for driving a micro mirror array tilts the mirrors with a torsion torque around the hinge, where the torque is generated by electrostatic force in a capacitive parallel plate configuration. An electrostatic attractive force is generated when two conductive plates at different electric potentials are brought in close vicinity with one another. The electrostatic torque is proportional to the square of the voltage difference across the gap between an electrode, the first plate, and the mirror, the second plate, and inversely proportional to the square of the gap size. The gap size varies as the mirror rotates through its range of angular positions. When the mirror is in its fully rotated orientation, the mirror is in a landing position. A mirror pixel of an SLM is switched from one state to another when the mirror is rotated from one landing position to another. The change of the state is accomplished by changing the net potential difference across the gap under both sides of the mirror hinge.
As the demand for SLMs with a large defection angle to achieve brighter images and higher contrast ratio increases, gap size is reduced drastically near the landing position. This can cause the electromechanical efficiency of the coupling between the two plates to deteriorate when transitioning from a parallel state to a large angle state. Traditionally, this can be compensated for by applying a higher bias or addressing voltage potential across the plates to ensure the operation of the SLM.
The optical properties of an SLM depends on the parameters of the micro mirrors, such as mirror fill-ratio, reflectivity, rotating angles, and angular transition times. However, the efficiency, robotics and reliability of the video operation also rely on the design of driving voltage waveforms. Two critical parameters from the physical property of the micro mirrors provide guidelines for CMOS control circuitry design. Snapping voltage, an indication of hinge stiffness, is a voltage reached when the electrostatic force is high enough that the mirror plate snaps from its quiescent or flat state to physically stopping at a landing position. Release voltage, an indication of surface adhesion or stiction, is a voltage reached when the electrostatic force is low enough that mirror is no longer held in contact with a landing stop and returns to its quiescent state.
In bi-directional operation, a pair of electrodes is positioned under the mirror plate on opposing sides the hinge. A third electrode connected to mirror plate is used in conjunction with the pair of electrodes to control the operation of the micro mirror arrays. One method to operate the bidirectional micro mirror array is to establish a fixed negative common bias on each mirror plate, then control the direction of rotation and enable the transition or rotation simultaneously by changing the voltage potential of the two addressing electrodes. The operation requires two independent transistor cells with complex circuitry. An alternative circuit design replaces one of the two transistor cells with an inverter so a single addressing voltage is sufficient to enable an angular cross over transition or rotation. The fixed negative common bias is used to lower the addressing voltage and compensate for the high snapping voltage of a typical MEMS based micro mirror.
Another method for a bi-directional operation is to control the voltage potential of an individual mirror plate by addressing circuitry while fixing the constant voltage potentials on each of the two bottom common electrodes on opposite sides of the pivot point of the hinge. This addressing scheme requires only a single drain line and one transistor per pixel, which significantly lowers the transistor count. However, each mirror pixel in the mirror array must be electrically isolated and individually and selectively addressable by an addressing circuitry in the control substrate. Another drawback is that the control circuit is vulnerable to the interference of photoelectron current generated by the incident light since the address node is directly connected to the mirror plate.